Removed Umbels Left at a Site Produce Viable Seeds

Two studies considered the possibility that cut-off umbels left at a site can produce viable fruits. The extent of this post-treatment fruit ripening is a warning. Pysek et al. (2007) show that 85% of terminal umbels cut off at the beginning of fruit formation produce some fruits - less and of lower quality than the control (18.6% in terms of number and 43.8% in weight), but nevertheless producing an average of 1840 fruits per plant of which 24% germinated. That is, although cutting umbels off at the stage of fruit formation in the terminal umbel reduces fecundity of newly produced umbels to less than 5% of the controls (see Table 7.2), this may be ineffective if the umbels are not removed from the site. However, since there is no principle difference between treatments applied at the peak of flowering and early fruiting (Fig. 7.2; Pysek et al., 2007), the former seems to be the better management strategy, because umbels cut at flowering are less likely to give rise to fruits.

The above study, however, did not determine the effect of the time of removal on post-treatment fruit ripening in removed umbels. To explore this issue, an experiment was performed at the Slavkovsky les research area in 2002 (for example, see Müllerová et al., 2005 and Perglová et al., Chapter 4, this volume, for details of the region). Umbels were removed at three pheno-logical stages: (i) late stigma receptivity; (ii) post-receptive; and (iii) fruit development, with ovaries already of a flat shape but not with final fruit size. Two treatments were applied at each stage: (i) only the terminal umbel was removed and left lying on the ground until harvest; or (ii) the flowering stem was cut at ground level and left at the site; in the latter case, the umbels remained attached to a cut stem. The removed umbels were enclosed in a fine mesh to prevent loss of ripe fruits and left on the ground within the H. mantegazzianum stands until the end of August (fruit ripening time) to simulate mechanical control. Although many fruits decayed in the wet microclimate within the ground vegetation layer, some fruits matured. The umbels that remained attached to flowering stems produced seeds, many of which germinated (Fig. 7.3). The percentage germination was not significantly affected by the timing of the treatment and even flowers cut as early as the end of stigma receptivity produced viable seeds provided they were connected to a stem (Fig. 7.3).

The type of treatment had a significant effect. A significantly higher percentage of seeds from umbels that remained attached to a stem germinated: 9.0% (max. 20.0%) if removed at the earliest phenological stage, 19.0% (max. 30.0%) at the later stage and 15.0% (max. 50.0%) at the fruit development stage. Corresponding values for isolated umbels were 0.0%, 1.7% (max. 10.0%) and 3.3% (5.0%), respectively (Fig. 7.3). A probable explanation is that fruits attached to stems are supplied with resources for an extended period following the treatment. It is also possible that the rigid stem kept some umbels above the soil level and away from the wet conditions.

These results further emphasize that timing of treatment is critical. From a management point of view, there is a 'trade-off' between the risk of fruit developing and reduction in fecundity. This trade-off is affected by the phenology of the plant. Very early removal of umbels results in high levels of regeneration. Removal of umbels later in a season results in a marked reduction in fecundity but the removed fruits are likely to provide viable seeds. In addition, when manipulating umbels later in a season, it is difficult to avoid ripe fruits being released.

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Late stage o[ stigma Post-receptivity stage Fruit development receptivity stage

Fig. 7.3. Percentage of seeds that germinated of those maturing in umbels cut off at three phenological stages. Two treatments were adopted: either the whole flowering stem (dark columns) or only umbels (light columns) were cut and left at the site. Deletion tests revealed a significant difference between treatments (F = 5.35, df = 1,12, P = 0.039) but no effect of the timing of removal. Umbels attached to the cut-off stems produced seeds that germinated better than that those produced by cut-off umbels. Based on original data.

Late stage o[ stigma Post-receptivity stage Fruit development receptivity stage

Fig. 7.3. Percentage of seeds that germinated of those maturing in umbels cut off at three phenological stages. Two treatments were adopted: either the whole flowering stem (dark columns) or only umbels (light columns) were cut and left at the site. Deletion tests revealed a significant difference between treatments (F = 5.35, df = 1,12, P = 0.039) but no effect of the timing of removal. Umbels attached to the cut-off stems produced seeds that germinated better than that those produced by cut-off umbels. Based on original data.

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